1. Field of the Invention
The present invention relates to remote fed intermediate regenerators for use in transmission lines for digital signals and in particular to a transmitting amplifier for such a regenerator.
2. Description of the Prior Art
Remote fed intermediate regenerators for use in transmission lines for digital signals having no direct current component are known in the art which contain for each transmission direction a series circuit including a distortion correcter, a regenerator circuit and a transmitting amplifier. The intermediate regenerator is preceded and followed by remote feed dividing filters. Each intermediate regenerator for each transmission direction further contains a high power Zener diode which bridges the entire remote feed path in the intermediate regenerator and the distortion correcter, the regenerator circuit and the transmitting amplifier each contain low power Zener diodes connected in parallel to the operating voltage terminals of those components for stabilizing the operating voltages thereof.
Such intermediate regenerators serve to regenerate and attenuate the transmitted pulses which have been sent via a transmission cable and to regenerate the position of the pulses in the time pattern. For this purpose the distortion correcter, located at the input end of the regenerator, cancels the frequency response of the transmission cable and is followed in series by the actual regenerator circuit. Following the regenerator circuit, at the output end of the intermediate regenerator, a transmitting amplifier is employed for producing the output signal having an amplitude necessary for the next section of the transmission link.
The low power Zener diodes used in the individual circuits within the regenerator exhibit a noticeable Zener bend so that the dynamic resistance of these low power Zener diodes is extremely small. In the event of excess currents the low power Zener diodes would thus absorb the excess currents until the diodes were destroyed without the voltage drop necessary to operate the high power Zener diode ever being reached. For this reason, a series resistor of relatively high impedence is inserted in the remote feed path, generally in the transmitting amplifier. The operation of this series resistor is such as to generate a considerable voltage drop in the event of excess currents so that the high power Zener diode will operate before the low power Zener diodes are destroyed. The series resistor must have a specific resistance value, or higher, in order to fulfill its protective function. The magnitude of the series resistor is, however, limited by the voltage drop which occurs during normal operation and which consequently represents a circuit loss. Generation of subsidiary voltages of this type is described in the Philips Telecommunication Review, Vol. 37, No. 3, Aug. 1979 at pages 161 through 169.
It is thus a problem in the art of dimensioning the series resistor to protect the low power Zener diodes without significantly increasing the circuit loss as a result of the presence of a high impedence.